1. Field of the Invention
This invention relates to a process for the chemical treatment of metal surfaces, in particular for phosphating or pickling, in which the metal surfaces are contacted with a solution of a process bath which comprises several components in aqueous solution, the composition of the process bath being maintained within a given range by addition of solutions or gases, in particular air.
2. Discussion of the Related Art
In the phosphating of metal sheets, these are contacted with so-called phosphating solutions by dipping into a process bath or by spraying the solution. A very thin layer of phosphate may form on the metal surface by this means, which serves, on the one hand, to protect against corrosion and, on the other hand, to improve the adhesion of a subsequently-applied lacquer. Thicker layers of phosphate are produced if these are to serve as a forming aid to facilitate forming processes. In addition to phosphoric acid, the solution comprises chiefly zinc, so that the metal surface is covered with a zinc phosphate layer. The phosphating solution also additionally comprises further components, e.g. further metal ions, such as manganese ions, and so-called accelerators, which ensure uniform formation of the phosphate layer on the metal surface. In contrast, phosphate layers which act as slip layers are, as a rule, based on manganese phosphate.
During running use of a given phosphating solution its composition changes, so that it must be topped-up from time to time or even continuously. The conventional phosphating solutions are, as a rule, acid, the pH being about 1.5 to 4.5. If the phosphating solution comprises acid-sensitive components, such as accelerators, which decompose rapidly in the acid phosphating solution, these components must be topped-up particularly frequently. For stability reasons, the acid-sensitive components are in an alkaline solution and are metered into the acid phosphating solution with this solution. The local increase in the pH to more than 4 resulting at the dropwise addition point is a disadvantage, leading to a precipitation of zinc ions as zinc phosphate, so that zinc or zinc phosphate must also be topped-up.
This applies to a greater degree if alkaline solutions or dispersions (for example sodium hydroxide solution or sodium carbonate solution, hydroxylamine, zinc carbonate) are added to the phosphating solution to buffer the free acid. Since these alkaline solutions greatly increase the pH at the point of addition, an increased amount of zinc phosphate precipitates as a sludge. With this formation of zinc phosphate sludge, the free acid increases again, so that renewed buffering must be effected. Since the valuable material zinc phosphate is lost from the phosphating solution as a result, this valuable material must be increasingly topped-up. This increases the operating costs of the process. On the other hand, the sludge may impede the phosphating process, so that it must be removed from the phosphating bath and disposed of. This also has an adverse effect on the cost of the phosphating process. The formation of sludge is particularly pronounced if alkaline solutions are added to the phosphating bath at a point were they are mixed only slowly with the phosphating solution. The effect of the increase in the pH, and as a result the sludge formation, could be reduced by faster mixing.
However, the composition of the phosphating solution changes not only due to a reduction in the content of its components. It is also undesirable if the content of iron ions increases too much. It is known to aerate the bath to decrease the iron content. For this, compressed air is introduced via fine-pored membranes arranged in the process bath, so that the oxygen in the air oxidizes the iron ions, which then precipitate and settle on the bottom of the bath container. The fine-pored membranes having a pore width of about 20 .mu.m produce very fine air bubbles which readily dissolve in the process bath. The consumption of the compressed air employed for the aeration, which is a significant cost factor, may be kept low by using such fine-pored membranes. However, considerable problems arise when the compressed air is switched off, since the fine membrane pores become blocked with phosphating sludge. The encrusted and blocked pores cannot be cleaned or may be cleaned only with considerable difficulty, so that, as a rule, the membranes have to be changed frequently. Furthermore, the usually high temperature of the phosphating solution, which may be up to 80.degree. C., leads to rapid wear of the membrane units.
The problems mentioned thus lead to increased costs of the phosphating process.
Similar problems exist in the topping-up of process solutions used for pickling high-grade steel and/or titanium and alloys thereof. These are both strongly acid and oxidizing, to break up and dissolve off surface deposits and to produce a uniform metallic surface. Such process solutions are described, for example, in EP-B-505 606. To maintain the reduction-oxidation potential, it is necessary to feed in an oxidizing agent, such as hydrogen peroxide or air, continuously or discontinuously.